libbsd: How to add an option for switching off IPv6 (or other BSD Kernel configurations)

[Christian Mauderer]

Hello,

we have a special use case where we want to use the new network stack
but without the IPv6 support. I could need some advice how this could be
integrated into the waf build system. If we find a good solution, this
could also be an example for other BSD Kernel options.


I found multiple places where IPv6 is enabled in libbsd:

If I remove the INET6 option out of the FreeBSD kernel configuration on
a FreeBSD system, this has the effect, that the "opt_inet6.h" is
generated as an empty file instead of with a single define "#define
INET6 1". In the rtems-libbsd the configuration header is located at
rtemsbsd/include/rtems/bsd/local/opt_inet6.h. It has a fixed content
with the define set. This would be the first place where the support
would have to be disabled.

Beneath that, some user space tools have a special option to enable IPv6
which is currently set in libbsd.py. One such example is tcpdump with a
compiler flag '-DINET6' set. As far as I could tell, the kernel and
every application that is included in libbsd currently uses the INET6
macro. But I think this is more of a convention than a general rule.


So I think we would roughly need the following:

If i configure libbsd using

   waf configure --disable-ipv6

the build system would either have to regenerate opt_inet6.h or use an
alternative version of it. Further I would need the ability to set
compiler flags in libbsd.py depending on this configure option.


Are there any better ideas how to implement such an option?

I'm quite inexperienced on how to use waf, so I would need some guidance
how this could be implemented. Are there any hints how I could start
implementing such an option? Some examples or similar code?

Kind regards

Christian Mauderer
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Christian Mauderer
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